The present invention relates to a method and a device for detecting and identifying bioaerosol particles in the air.
Such a method and a test arrangement used therein are known from an article by M. A. Stowers et al.: Application of matrix-assisted laser desorption/ionization to on-line aerosol time-of-flight mass spectrometry, Rapid Commun. Mass Spectrom. 14, 829-833 (2000). This article indicates that these are two options for xe2x80x9cquasi-real-time bioaerosol detectionxe2x80x9d, namely a method in which the intrinsic fluorescence of biomolecules is utilized and methods in which mass spectrometery is applied to ions generated by laser desorption. Because bioaerosol particles form only a small part of the total content of aerosol particles in the air, the known xe2x80x9csingle particlexe2x80x9d fluorescence detectors may be used to indicate whether the aerosol particles are of biological origin or non-biological origin; they are, however, not arranged for identifying bioaerosol particles. The above article therefore describes an identification experiment which starts from bioaerosol particles in a liquid which is nebulized, after which the spray is drawn into an ATOFMS (aerosol time-of-flight mass spectrometer). In this ATOFMS an aerosol particle identification is carried out on the basis of MALDI (matrix-assisted laser desorption/ionization). Even though for this experiment an aerosol has been prepared first, the object of the study indicated in this article is still directed to an identification carried out on real-time basis. In the manner described, however, this cannot be realized in a satisfactory manner, because the air for the greater part contains non-bioaerosol particles. The ATOFMS detection and identification apparatus used is therefore relatively insensitive to bioaerosol particles. In view of the fact that a rapid detection and identification of bioaerosol particles is absolutely necessary under specific conditions, for instance under conditions of biological warfare, the object of the present invention is directed to the aim stated in the above article, namely a rapid on-line bioaerosol particles detection and identification.
A rapid on-line aerosol particle detection and identification, not relating to bioaerosol particle detection and identification, is, for that matter, already known from the international patent application WO96/31900. According to this application an air stream containing particles is passed through an ATOFMS system. A rapid detection and identification of specifically bioaerosol particles is not possible in this manner, because, as mentioned before, there will be relatively few bioaerosol particles in the air, and precisely these particles, as soon as they are spread in the air, must be detected and identified.
In order to solve the above-mentioned problems, in the method according to the invention the bioaerosol particles in a particle stream are selected in an ATOFMS by means of fluorescence techniques, after which successively the selected bioaerosol particles are ionized, the resulting ions are detected and the bioaerosol particles are identified. For the selection of bioaerosol particles the known per se property is utilized that the presence of specific substances, such as, for instance, amino acids, when irradiated with a suitable wavelength, induces a characteristic fluorescence. Thus, the irradiation of tryptophan by UV laser light of 266 nm gives a broad fluorescence in the wavelength range of 300 to 400 nm. Also eligible are the fluorescence spectra of substances, such as tyrosine, NADH or riboflavin (see: Fell et al.; Concentration, size, and excitation power effects and microparticles containing tryptophan and bacteria, SPIE, vol. 3533, pp. 52-62). In general, inorganic and most of the organic substances do not show this characteristic. In order to utilize this property in the method according to the invention, the selection of bioaerosol particles takes place by means of laser radiation, generated by a first laser device, of a wavelength which in specific substances in bioaerosol particles effects a fluorescence, after which by means of a detector for detecting the fluorescence radiation the bioaerosol particles are selected, and a second laser device is triggered to emit light of a wavelength which effects the ionization of the bioaerosol particles selected only by the fluorescence detector. For the first laser device a continuous-wave laser device (cw-laser device) is preferably used, while for the second laser device a pulse laser device is used, the pulses being triggered by the fluorescence detector.
It is efficient in the selection of bioaerosol particles to also utilize the size, that is to say the aerodynamic size, of the aerosol particles. The size of bacteria and viruses is substantially in the range below 20 xcexcm. Because the aerosol particles enter the central space of the ATOFMS at a given speed, the size of the successive aerosol particles can be determined from the duration of a known distance traversed by an aerosol particle. By directing the laser beam of the first laser device to two successive spots with a known mutual distance, the above duration and hence the size of the aerosol particle can be determined from the light scattered and detected by an aerosol particle. However, the distance between the spots must then be smaller than the mutual distance of the successive particles. For instance, at a distance between the spots of 2.5 mm and a speed of the aerosol particles of approximately 400 m/s the duration between two measurements is approximately 6.25 xcexcs. To obtain the desired unambiguity between measurements on the above spots, the first laser device is, according to a further aspect of the invention, a two-color continuous-wave laser device, operative at wavelengths of 266 and 532 nm. The wavelength of the light directed to the first spot is 532 nm, and that of the light directed to the second spot is 266. This last wavelength is also such that bioaerosol particles then show fluorescence.
The second laser device is, for instance, an Excimer laser device, operative at a wavelength of 308 nm and, after a proper triggering, ensuring the ionization of only the bioaerosol particles. To this end, the pulses of this laser device, as mentioned before, are triggered by the fluorescence detector. The use of a UV Excimer laser implies that in order to enable the performance of the method described herein on the basis of MALDI, the aerosol particles must be provided with a matrix. According to a further aspect of the invention, the aerosol particles, during or immediately before drawing into the ATOFMS, are therefore provided with such a matrix by evaporation/condensation or sublimitation/condensation. Should an IR (infrared) laser device be used, then this is not necessary.